This invention relates to a method of forming a film of a carbon nano-tube on a substrate, and particularly, to a method of preparing a film of a carbon nano-tube which enables the use of the carbon nano-tube for a field emission source.
A field emission source has the many advantages of, for example, energy saving, prolonged lifetime compared with an electron source making use of thermal energy, that is a thermion emission source. At present, tungsten, silicon, molybdenum and so on are known for the materials of the field emission source.
The field emission source should have an acute tip to which a field is concentrated. However, it is not easy to work acutely the tip of metal materials such as tungsten and so on. And, it is also necessary to maintain the inside of an electron tube in high vacuum of the order of 10xe2x88x928 Torr and over in order to keep acuteness of the tip of the field emission source while using. Thus, it is not only extremely difficult to produce the field emission source using a metal material, but also the subsequent production of the electron tube is made difficult.
Recently, a carbon nano-tube free from the aforementioned disadvantages has been noted for a material of the field emission source. A carbon nano-tube has been considered to be a field emission source of great promise, because of its characteristics that it in self has acuteness enough to concentrate a field, and is chemically stable, as well as is mechanically tough.
The carbon nano-tube may be roughly classified into two groups, one is a multi-wall carbon nano-tube (MWNT), and the other is a single-wall carbon nano-tube (SWNT). The multi-wall carbon nano-tube comprises, as shown by its name, two and over concentric cylindrical tubes and the tip of which is closed. The single-wall carbon nano-tube comprises a cylindrical tube of one wall, and the tip of which is open. Above all, the multi-wall carbon nano-tube is used for the field emission source.
The multi-wall carbon nano-tube can be obtained by carrying out DC arc discharge under a gas atmosphere using a pair of pure carbon electrodes. That is to say, an anode carbon electrode is vaporized by arc discharge to form soot and cathode deposit containing a multi-wall carbon nano-tube.
It has been reported by Collins et al. that the multi-wall carbon nano-tube which is not separated and purified is solidified with epoxy resin to form a good field emission source (P. G. Collins et al. Appl. Phys.Lett 69 (13), Sep. (1996). p.1969). However, it has been confirmed by Smally et al. that the multi-wall carbon nano-tube the tip of which is opened by separation and purification has the important advantages as the field emission source of low threshold, high electric current density and so on (Smally et al. Science vol.269, 1550 (1995)). The separation and purification are carried out as stated below.
First, a crude multi-wall carbon nano-tube is pulverized in a mortar. Next, the pulverized crude multi-wall carbon nano-tube is dispersed in ethanol, and is subjected to ultrasonic wave. Then, ethanol in which the crude multi-wall carbon nano-tube is dispersed is filtered and filtrate is dried. The substance thus obtained is sieved and only the substances which pass through a screen are heated to burn by a burner on a quartz glass plate.
As explained above, the separated and purified multi-wall carbon nano-tube is not only heightened in its purity, but also shows good properties as the field emission source since its tip is open.
In order to use the multi-wall carbon nano-tube as the field emission source, it is preferable to deposit the multi-wall carbon nano-tube as a film on a substrate, that is, to form a film-like multi-wall carbon nano-tube, and a method therefore was developed by De Heur et al., a Swiss researcher of atom cluster (SCIENCE 268 (1995) 845). The method is that the multi-wall carbon nano-tube which has been previously separated and purified is passed through a porous ceramic filter of 0.2 xcexcm in diameter and then is transferred on Teflon or aluminum foil. And, the results of evaluation of the field emission source making use of the film-like multi-wall carbon nano-tube thus obtained have been reported in SCIENCE 270 (1995) 1179.
The recovery of the multi-wall carbon nano-tube is only 10% something on the basis of consumption of raw materials. The separation and purification of the multi-wall carbon nano-tube are troublesome. Therefore, the field emission source using the multi-wall carbon nano-tube has disadvantages that it is not suitable to mass production because of high cost.
The method of preparing a film of carbon nano-tube according to De Heur et al. has also a disadvantage that it can not be applied to production of a film of a single-wall carbon nano-tube for reason that the single-wall carbon nano-tube can not be trapped in small holes of a ceramic filter since the single-wall carbon nano-tube has characteristics that it is rich in flexibility and is apt to be intertwined to form bundles.
An objective of this invention is to provide a method of preparing a film of a carbon nano-tube which is able to omit trouble with the separation and purification of the carbon nano-tube and is able to provide a cheap field emission source.
Another objective of this invention is to provide a method of preparing a film of a carbon nano-tube which is suitable for a method of preparing a film of a single-wall carbon nano-tube.
According to a first aspect of this invention, there is provided a method of preparing a film of a carbon nano-tube, characterized in that a crude carbon nano-tube is dispersed in a solvent, an ultrasonic wave is applied thereto, and thereafter the aforementioned solvent is evaporated to deposit the carbon nano-tube on a substrate placed in the aforementioned solvent.
According to a second aspect of this invention, there is provided a method of preparing a film of a carbon nano-tube, characterized in that the aforementioned crude carbon nano-tube is dispersed in the aforementioned solvent, thereafter precipitates are removed, and then the carbon nano-tube is deposited on the aforementioned substrate by making use of supernatant suspension.
According to a third aspect of this invention, there is provided a method of preparing a film of a carbon nano-tube of the first or second aspect of this invention, characterized in that the carbon nano-tube is deposited on the aforementioned substrate by using filtrate which is obtained by filtering the aforementioned solvent in which the aforementioned crude carbon nano-tube is dispersed or the aforementioned supernatant suspension.
According to a fourth aspect of this invention, there is provided a method of preparing a film of a carbon nano-tube of the first, second or third aspect of this invention, characterized in that the aforementioned crude carbon nano-tube is pulverized and thereafter dispersed in the aforementioned solvent.
According to a fifth aspect of this invention, there is provided a method of preparing a film of a carbon nano-tube of the first, second, third or fourth aspect of this invention, characterized in that the aforementioned substrate is a conductive plate such as a metal plate, a carbon sheet and so on.
According to a sixth aspect of this invention, there is provided a film of a carbon nano-tube prepared by the method of the first, second, third, fourth or fifth aspect of this invention.
According to a seventh aspect of this invention, there is provided a field emission source using a film of a carbon nano-tube prepared by the method of the first, second, third, fourth or fifth aspect of this invention.